Sports shoes used for various competitions are composed of a lot of members. For example, a shoe sole is composed of shoe sole members such as an outer sole, a midsole, and an inner sole.
Materials used for such shoe sole members are required to be lightweight and to have properties such as mechanical strength and shock absorbing properties for reducing deformation due to a long-term use and imparting durability under severe use conditions.
Therefore, the shoe sole members are formed by a foam mainly composed of a crosslinked polymer.
Conventional shoe sole members are formed by a foam obtained by crosslinking and foaming polyurethane, natural rubber, or ethylene-vinyl acetate copolymer, from viewpoints such as durability, cost, and constraints on production. Particularly, shoe sole members formed by a crosslinked foam of ethylene-vinyl acetate copolymer are widely used (see Patent Literature 1 below).
However, conventional crosslinked foams have a problem of being difficult to recover their original shape when a compressive strain is applied for a long time, and having such a phenomenon being increased particularly when they are highly foamed.
Therefore, in the case where a crosslinked foam that is more highly foamed than conventional products for exhibiting lightness of weight is applied as a forming material, conventional shoe sole members may possibly be unable to sufficiently recover after use from the compressive deformation during the use.
A shoe sole member having excellent recovery properties from the compressive deformation can be obtained generally by applying a foam formed by a resin having high rigidity as a forming material.
However, the hardness of a foam formed by a polymer with high rigidity tends to increase to a hardness higher than necessary, as compared to a foam formed by a soft polymer with low rigidity, which makes it difficult to allow shoes to exhibit sufficient comfort.
That is, conventional shoe sole members have a problem of being difficult to provide products that are light and soft, and exhibit excellent comfort during use and excellent recovery properties after the use from the compressive deformation applied during the use.